The present invention relates to a spark plug for a spark-ignited internal combustion engine, and more particularly toward a spark plug having a hermetic seal about a lower center electrode constructed using a lead-free sealing material.
A spark plug is a device that extends into the combustion chamber of an internal combustion engine and produces a spark to ignite a mixture of air and fuel. In operation, electrical potentials of up to about 40,000 volts are applied through the spark plug center electrode, thereby causing a spark to jump the gap between the center electrode and an opposing ground electrode.
Internal resistor/suppressor spark plug designs are well-known and have proven successful over many years. A key element of the successful spark plug design is a hermetic seal between the center electrode and the internal bore of the insulator element of the spark plug. During operation in an engine, the distal portion of the insulator and center electrode will be exposed to the high pressures generated in the combustion chamber, as well as the elevated temperatures generated by the combustion process. A hermetic seal, capable of withstanding the temperatures and gas pressures generated in the combustion chamber over the spark plug lifetime, is essential. The seal must prevent the intrusion of hot, pressurized gases from the combustion chamber into the upper portion of the spark plug, as such gases will damage the discrete resistor/inductor etc. elements and render the spark plug non-functional. In extreme cases, damage may also occur to other related portions of the spark plug (terminal stud) and the engine generally (spark plug wires, etc.).
An exemplary spark plug is illustrated in FIG. 1 having a hermetic glass seal between the insulator and center electrode assembly. The use of such glass seals is well known in the industry as the glass seal provides a hermetic seal to prevent the escaping of various gases from the combustion chamber. Until recently, the glass used in between the insulator and center electrode assembly contained large portions of lead or lead oxide such as taught by U.S. Pat. No. 2,898,395 to Schurecht. In some spark plugs, the lead oxides are 50% by weight or more of the glass sealing material. The use of such substantial amounts of lead is technically desirable, as such leaded glass improves the workability and robustness of the glass seal material and the spark plug. A glass seal material with various lead-containing glasses provided benefits during the manufacturing process including improved fusing of the glass to the surrounding ceramic materials and metal wires at low sealing process temperatures and robust and reliable bonds to ceramic materials. Furthermore, leaded glass bonded well to metals. Another benefit of lead-containing glasses was that as the glass powder was heated, the viscosity of the glass varied minimally with temperature in the process temperature ranges allowing for variability in the manufacturing process without detrimental effect to the resulting spark plugs. All of the above benefits of leaded glass allowed for easy manufacturing of reliable spark plugs.
Due to recent environmental and safety restrictions and legislation in the automotive and glass industries, the use of leaded materials in spark plug construction is no longer desirable, despite the advantages cited. The changes in environmental and safety restrictions have also caused significant increase in the cost use of leaded glass materials. Therefore, most spark plug manufacturers are trying to phase out or limit the amount of lead used in hermetic glass seals between the insulator and center electrode. Therefore, a lead-free glass which: (1) may be processed at temperatures similar to leaded glass, (2) is robust with respect to processing, (3) bonds well to the metals and insulators used in construction of the seal, and (4) uses processes similar to existing process is desirable.